1. Field of the Invention
The present invention relates generally to the implementation of dynamic switch protocols on a shared medium network. More particularly, the present invention superimposes dynamic switch protocols on a token bus protocol to improve the basic token bus functional capabilities and link utilization, and to produce a uniform transaction protocol that supports both token bus and dynamic switch networks.
2. Discussion of the Prior Art
A token bus is well known in the prior art, and is characterized by a physical broadcast medium in which all nodes receive every transmission and in which only one node may transmit at any time. Token buses may be applied to classical electrical bus structures, radio transmissions, and in the case of fiber optical media, to passive star optical couplers.
Transmissions on a token bus are structured as frames which are addressable to the various other nodes on the token bus. Ordinarily, in a token bus architecture, a single token is circulated among all the nodes on the bus to control access to the bus medium; that is, at any time only one node on the bus will hold the token and thereby own the bus medium for transmission purposes. When a node completes its transmissions, it transmits the token to the next node in the token circulation policy. Typically, the token circulates to each node on the bus once in a cycle, forming a logical access ring.
IEEE 802.4 is a token bus architecture that has been approved as an industry standard by the American National Standards Institute (ANSI) since December 1984, and reference can be made to the fourth edition of the IEEE 802.4 architecture specification, published in June of 1988 by the IEEE.
The IEEE 802.4 architecture describes a basic token-passing media access protocol for a shared physical bus medium. In one embodiment, the present invention superimposes dynamic switch protocols on the IEEE 802.4 token bus protocol to enhance the efficiency of that protocol.
The IEEE 802.4 token bus standard is representative of token bus architecture, and defines protocols to initialize the logical ring, exchange the token, and recover the token if the token is lost. All transmissions within IEEE 802.4 are fully encapsulated frames with complete link headers. The IEEE standard defines two frame transmission protocols: non-interlocked and request-with-response. Non-interlocked frame transmissions are simple single frame transmissions to an arbitrary destination. Request-with-response transmissions allow a single frame transmission to a destination that then implicitly acquires the sender's token rights to provide a single frame response to the sender's request frame. Upon completion of the destination's response frame transmission, the original requester resumes its token rights. In both the non-interlocked and request-with-response protocols, the frame transmissions carry a complete link header with control and destination addressing information.
The present invention alleviates a major source of inefficiency associated with the above transmission protocols: Attaching a complete link header to every transmitted frame results in unnecessary overhead when a large block of data is transmitted as a sequence of smaller frames. Such transmissions are necessary because of buffer-space limitations in the stations and also to achieve better error-coverage. For example, with 256-byte data frames and 32-byte headers, the overhead introduced is approximately 12.5 percent of the total transmission. The transmission protocols implemented by the present invention reduce this overhead by allowing a virtual circuit to be established between two link servers.
The token-bus is a blocking medium, and therefore allows only one transmission to proceed in the network at any time. The performance of the network can be improved by the substitution of a non-blocking switch in place of a passive coupler without affecting the topology of the network. The use of a non-blocking switch affords higher throughput and enhanced functional capabilities. Although IEEE 802.4 transmission protocols can be applied to a switched-star network which utilizes a non-blocking switch, the resulting implementation is relatively inefficient, and does not effectively utilize the additional functional capabilities of the non-blocking switch. It is desirable to define uniform transmission protocols for these two network-types, the switched-star and the passive-star, so that one type can be substituted for the other without the need to alter the frame formats; this allows the use of identical frame-formatting and recognition hardware to be used in both cases, greatly facilitating a common link-server design. Such inter-operability is a prime motivating factor behind the present invention.